[llvm] [MemCpyOpt] Drop dead `memmove` calls on `memset`'d source data (PR #101930)

[Yingwei Zheng via llvm-commits]

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@@ -1854,12 +1855,53 @@ bool MemCpyOptPass::processMemCpy(MemCpyInst *M, BasicBlock::iterator &BBI) {
   return false;
 }
 
+/// Memmove calls with overlapping src/dest buffers that come after a memset may
+/// be removed.
+bool MemCpyOptPass::isMemMoveMemSetDependency(MemMoveInst *M) {
+  MemoryUseOrDef *MemMoveAccess = MSSA->getMemoryAccess(M);
+  if (!MemMoveAccess)
+    return false;
+
+  BatchAAResults BAA(*AA);
+  MemoryAccess *FirstDef = MemMoveAccess->getDefiningAccess();
+  MemoryLocation SourceLoc = MemoryLocation::getForSource(M);
+  MemoryAccess *SourceClobber =
+      MSSA->getWalker()->getClobberingMemoryAccess(FirstDef, SourceLoc, BAA);
+
+  MemSetInst *MS = nullptr;
+  if (auto *Def = dyn_cast<MemoryDef>(SourceClobber))
+    MS = dyn_cast_or_null<MemSetInst>(Def->getMemoryInst());
+
+  if (!MS || MS->getParent() != M->getParent())
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dtcxzyw wrote:

Please add a negative test for `MS->getParent() != M->getParent()`. Why we cannot do this if the memset and memmove are not in the same block?

https://github.com/llvm/llvm-project/pull/101930